Aedes excrucians (Walker)

by Wayne J. Crans, Rutgers University


Subgenus : Ochlerotatus

Type of Life Cycle : Univoltine Northern Aedes

Typical Habitat : Moderately deep snowpools in forested areas

Larvae Present : Early Spring

Head Hairs

Upper: Double (Occasionally single or triple)

Lower: Double (Occasionally single)

Antenna

Length: Shorter than head

Tuft: Inserted before middle of shaft

Abdominal Hairs (Segments III-VI) : 1-1-1-1

Comb Scales : Patch

Siphon

Index: 5.0, tapered and very slender at apex

Tuft: 3-7 Hairs

Pecten: 1-3 teeth detached

Anal Segment

Saddle: Incomplete

Precratal tufts: 3-5

Other : 1) Pointed gills that are very light in color

2) Tracheation very obvious in the gills

GEOGRAPHIC DISTRIBUTION: Aedes excrucians is a mosquito that reaches greatest numbers in forested areas of northern North America. The mosquito's range includes the northern 1/3 of the United States northward through most of Canada and Alaska. The southern limit extends on a line from New Jersey on the east coast to southern Oregon in the west. Populations in the higher elevations of the mountain states extend as far south as northern New Mexico. Aedes excrucians is abundant in the northern counties of New Jersey and larvae are common in suitable habitat from High Point in Sussex County to the Great Swamp area of Morris County. Isolated populations extend into the coastal plain but collection records indicate that the mosquito is not common in the southern half of the state.

SEASONAL DISTRIBUTION: Aedes excrucians is a univoltine species with a typical northern Aedes life cycle. The single generation of eggs hatch early in the season and the larvae develop during the month of April. In the southern portion of its range, larvae may enter 4th instar as early as the second week of April. In higher elevations of the more northern counties, development is slower and larvae can be collected into early May. Occasional specimens lag behind the main brood and linger in larval habitats in small numbers. In most cases, Ae. excrucians larvae that are slow to pupate show evidence of Vorticella parasitism and probably die before they enter the pupal stage. The adults are on the wing during May and the females lay their eggs around the borders of snow pools that are rapidly drying down as the trees draw heavily on available ground water. The embryos enter an obligatory diapause and do not hatch until the following spring regardless of how many times the pools flood during the summer months.

LARVAL HABITAT: Aedes excrucians can be found in a variety of early season habitats but the species is most common in snow pools lined with heavy leaf litter. The species is usually found in moderate to deep pools but specimens can be collected from shallower breeding depressions that retain their level through spring water runoff. In northern Sussex County, the species is frequently mixed with Aedes communis but is always at least 2 instars behind. In the southern portion of its range, the species may be found with Aedes grossbecki in woodland pools located in lowland forests of Red Maple and Sweet Gum. Very dark habitat water is common at both the northern and southern extremes of its range.

COMMON ASSOCIATE SPECIES: Ae. communis, Ae. canadensis, Ae. stimulans, Ae. grossbecki

LARVAL COLLECTION: Early instar Ae. excrucians larvae congregate within stands of emergent grasses along the borders of the breeding habitat and dipping directly into a grassy tuft can produce much larger numbers than random dips taken throughout the pool. Once the larvae reach 3rd instar, they disperse and spend a great deal of time below the surface. In deep pools, submerged Ae. excrucians larvae can be located by slowly moving the dipper through the water and using the white surface as a background. The larger larvae stand out clearly and can be isolated and collected by slowly raising the dipper under the swimming specimen. As the season progresses, the pools become shallower and moving the dipper through the water stirs up too much litter to be effective. The habitat water usually darkens as the pool dries down and the larvae resting on the bottom cannot be seen clearly. If the pool is disturbed, the larvae burrow deep into the leaf litter making collection extremely difficult. Random dipping along the edge is usually more productive in dark water habitats during the later portion of April. If the dipper is forced into the leafy border at the edge of the pool, enough suction can be generated to draw larvae and pupae from their hiding places in the shallow litter. Larvae may be more numerous in the deeper sections of the pool but their habits make collection difficult.

LARVAL IDENTIFICATION: Aedes excrucians larvae are unique and relatively easy to recognize in the field. The 4th instar is quite large and the long slender air tube is a good character to look for when the specimen is still in the dipper. In the laboratory, moderate magnification reveals detached pecten teeth, which separate the species from Aedes fitchii, the only other early season Aedes with a similarly tapered air tube. Early instar Ae. excrucians larvae have single head hairs which do not match couplet characteristics for this species in most keys. Larval keys are designed for 4th instar specimens and early instar Ae. excrucians are frequently misidentified because of the head hair discrepancy. Later instars larvae develop the normal 2 upper and 2 lower head hair pattern, and 3 upper head hairs is not uncommon in some populations.

REPRESENTATIVE COLLECTION RECORDS:

Northern New Jersey

Location: High Point State Park, Sussex Co.

Date: April 14

Habitat : Snow Pools on Ridge Road

Instar: 2nd & early 3rd

Central New Jersey

Location: Great Swamp National Wildlife Refuge, Morris Co.

Date: April 19

Habitat : Semi-permanent Woodland Pools

Instar: 4th & Pupae

Southern New Jersey

None collected

IMPORTANCE: Although Ae. excrucians is common in the northern portion of its range, it does not appear to function as a significant pest in new Jersey. The species has an irritating bite but the mosquito does not fly far from its breeding habitat and, as a result, causes very local annoyance. Separating this species from the more numerous, Aedes stimulans, is difficult in the adult stage. As a result, some of the annoyance attributed to Ae. stimulans (which is regarded as a pest in Sussex ,Warren and Morris Counties) may, in part, be due to adults of this species. Aedes excrucians adults are extremely long lived and specimens can be collected into the month of August. There is evidence that the mosquito plays a role in dog heartworm transmission in some areas of the northern United States.


Center for Vector Biology